SummaryAmidolytic assays for Factor X with the chromogenic substrate S-2337 were found to be accurate and reproducible. Reference values in a normal population were 8.7–17.4μkat 1-1with a mean values of 13.1±2.2μkat 1-1; the distribution is normal with a statistically significant difference between males and females.A reasonable correlation was obtained between prothrombin time values and amidolytic Factor X determinations in 44 patients on oral anticoagulant therapy (correlation coefficient 0.73). Fully concordant results were obtained in 73 percent of the samples whereas fully discordant results were not observed. Prothrombin times between 15 and 25 % of normal (ratio between 1.7 and 1.3) corresponded to Factor Xa levels between 3.5 and 5.1μkat 1-1(27 to 39 percent of the reference value). Fully concordant results were obtained in 88 percent of the samples of patients treated with fenprocoumon (n = 34) but only in 40 percent of the patients treated with acenocoumarol (n = 10).Factor X measurements were found to be equally sensitive as prothrombin time assays to small decreases of vitamin K-dependent factors in patients with liver disease.